Winders and rewinders are machines that roll lengths of paper, such as tissue paper, into rolls. A winder is typically known as an apparatus that performs the very first wind of the paper web, forming what is generally known as a parent roll. A rewinder, on the other hand, is typically known as an apparatus that unwinds the parent roll into smaller rolls that represent the finished product. For instance, in one embodiment, a parent roll of bath tissue can be unwound in a continuous fashion by a rewinder and fed into a process by which the paper web is wound onto cores supported on mandrels to provide individual, relatively small diameter logs. The rolled product log can then be cut to designated lengths into the final product. In addition to toilet tissue rolls, other final products that can be made by this process include paper towels, paper rolls, and the like.
To conserve bulk in the finished product, especially when producing toilet tissue rolls and paper towel rolls, the parent rolls may be wound somewhat loose. Typically, the parent rolls are moved to storage locations until they are consumed in a converting process during which the final products are made. The handling and storage of the parent rolls can subject the rolls to certain stresses that cause the rolls to become disoriented from a pure cylindrical shape. Storing a parent roll on a hard surface, for instance, can cause a flat spot on the roll. Such rolls can have an elliptical or eccentric shape depending upon how the roll is handled.
As the rolls are unwound by a rewinder, any out-of-roundness characteristics may cause tension disturbances within the sheet material. These tension disturbances can cause many problems. Differences in tension in the web as the web is fed into a process can cause machine malfunctions, web breaks, and can lead to the production of non-uniform final products.
In the past, in order to control tension fluctuations, dancer rolls were inserted into the process between first and second sets of driving rolls or between first and second nips. The basic purpose of a dancer roll is to maintain constant tension on the continuous web as the web is fed into a downstream process and traverses a span between first and second sets of driving rolls.
As the web traverses the span, passing over the dancer roll, the dancer roll moves up and down in a track, serving two functions related to stabilizing the tension in the web. First, the dancer roll provides a damping effect on intermediate term disturbances in the tension in the web. Second, the dancer roll temporarily absorbs the difference in drive speeds between the first and second sets of driving rolls, until such time as the drive speeds can be appropriately coordinated.
Typically, the dancer roll is suspended on a support system, wherein a generally static force supplied by the support system supports the dancer roll against an opposing force applied by the tension in the web and the weight of the dancer roll. So long as the tension in the web is constant, the dancer roll remains generally centered in its operating window on the track.
When the web encounters an intermediate or long term tension disturbance, temporarily increasing or decreasing the tension in the web, the imbalances of forces on the dancer roll cause translational movement in the dancer roll to temporarily restore the tension, and thereby the force balance. So when difference in the speeds of the first and second sets of drive rolls tend to accord a change in the web tension, the dancer roll temporarily maintains the tension.
Thus, the dancer roll generally stabilizes the tension in the web, by compensating for temporary changes in the operating tension. While the dancer roll, as conventionally used, provides valuable functions, it also has its limitations.
Examples of dancer rolls are described in U.S. Pat. No. 5,659,229 and in U.S. Pat. No. 6,856,850, which are both incorporated herein by reference. The dancer roll disclosed in the '229 patent is an active roll in which active and variable forces are applied to the dancer roll. The system and method disclosed in U.S. Pat. No. 5,659,229 have provided great advances in the art.
Further improvements are still needed, however, in the ability to control web tension. For instance, a need still remains for a device for controlling web tension that has fast response times, especially when the paper web is moving at high speeds.